The present invention relates to heat-curable aqueous coating compositions for use on metallic surfaces.
Such compositions are based on a mixture of a binder and various optional ingredients such as organic solvents, pigments, fillers, the customary auxiliary agents and additives etc. The binder in the compositions of interest comprises:
(A) 60-95% by weight of a water-soluble and/or water-dispersible addition compound of 10-35% by weight of one or several .alpha.,.beta.-unsaturated dicarboxylic acids and 65-90% by weight of a butadiene polymer liquid at 20.degree. and having at least 50 mole% of butadiene units and a molecular weight in the range of 400 to 6,000, the addition compound being at least partially neutralizable with basic compounds and esterified,
and
(B) 5-40% by weight of a water-soluble and/or water-dispersible crosslinking resin having, on the average, at least 2 reactive groups per mole.
Such coating compositions are conventional and disclosed, e.g., in DAS [German Published Application] 2,120,962 and DOS [German Unexamined Laid-Open Application] 1,720,576. Although possessing generally acknowledged advantages, the aforementioned binders also exhibit specific deficiencies regarding their processing and/or attainable film properties. Thus, by combining heat-reactive formaldehyde resins of phenolcarboxylic acids with partially esterified, carboxy-group-carrying butadiene polymers, aqueous coating compositions can be obtained having a good stability during storage and application procedure and which produce coatings having increased hardness. However, in combination with binders based on polybutadienes of a high cis-1,4-content, which offer good corrosion protection when used alone as binders, these crosslinking resins result, with increasing amounts, in a rising rust undermining of the coated films in the salt spray test.